<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var a = prompt("请输入分数");
        if (a >= 90) {
            alert('A');
        } else if (a >= 80) {
            alert('B');
        } else if (a >= 70) {
            alert('C');
        } else if (a >= 60) {
            alert('D');
        } else {
            alert('E');
        }
    </script>
</head>

<body>

</body>

</html>